1. Field of the Invention
The present invention relates to an apparatus for reproducing digital data, such as a digital video tape recorder (digital VTR).
2. Description of the Prior Art
Since the amount of information to be recorded is great in a digital VTR, four magnetic heads, for example, are disposed on a drum at an axial interval corresponding to a track pitch individually, and four recording tracks are formed on a magnetic tape per rotation of the drum.
The video data of one field is divided into four channels, and the divided data of the individual channels are recorded with two rotations of the drum. With regard to tracks recorded on the magnetic tape, the data segment of each track is divided into a plurality of blocks, and there are added to each block a synchronizing signal, identifying signals representative of an even or odd field, a segment number and a recording head, and a block address. However, an identifying signal of the same content appears at every 16 tracks, for example. Accordingly, when writing the data in a video memory incorporated in each channel data processing circuit, it becomes necessary to accurately detect and recognize the state of the identifying signal for achieving exact recovery of the data.
However, there may occur a phenomenon that a magnetic head of one channel picks up the data from the track recorded by a magnetic head of another channel, and such phenomenon is unavoidable particularly in a varied-speed reproduction mode with occurrence of continuous positional deviation.
As a result, there also exists a case where the data not corresponding to a certain channel is written in the video memory incorporated in the channel data processing circuit of that channel and consequently the desired video data fails to be reproduced.
Regarding this problem, there may be contrived a technique of first detecting the positional deviation of a magnetic head from a recorded track (difference between one track relative to a magnetic head and another track being scanned by the same magnetic head; hereinafter referred to as shift value) in accordance with the identifying signal, then switching the channels of the input data and writing the data in the predetermined video memory.
However, in a varied-speed reproduction mode where each magnetic head executes its scanning while traversing the adjacent tracks, the envelope level of the pickup signal may be lowered to cause a problem that the identifying signal cannot be read out with accuracy. Accordingly, it becomes impossible to achieve precise detection of the shift value, hence failing in solution of the problem with regard to attaining satisfactory video data reproducibility.